1. Technical Field
The present invention relates to a droplet ejecting apparatus, and particularly to a droplet ejecting apparatus suppressing occurrence of nozzle clogging. The invention also relates to a cap to be provided at a liquid ejection port in an ink jet system and a droplet ejecting apparatus having the cap, and particularly to a cap and a droplet ejecting apparatus suppressing occurrence of clogging of a liquid ejection port due to drying.
2. Related Art
Ink jet systems for ejecting liquid from nozzles comprising nozzles, slits and porous film are used widely in various printers because of their small size and low price. Among ink jet systems, a piezo ink jet system for ejecting ink by making use of deformation of piezoelectric elements, and a thermal ink jet system for ejecting liquid by boiling a liquid using thermal energy are noted for excellent characteristics such as high resolution and high speed printing.
When the liquid is ink, generally, water-based inks containing mainly water as a solvent, and oil-based inks containing mainly an organic solvent as a solvent are known. Water, which is used as a solvent in a water-based ink, is high in vapor pressure, and thus is evaporated with the lapse of time, which is problematic since the coloring material in the ink then solidifies.
Ink jet systems can also be applied to formation of a circuit pattern or a color filter by ejecting curable resin. In this case, however, the viscosity of the liquid including the curable resin may be increased by drying, whereby problems may occur in ejection.
Thus, in the ink jet system, the ejection stability of an ink jet tends to be influenced by the external environment, and nozzle clogging may occur with the passage of time.
To overcome such problems, it has been proposed to dispose vapor generating means between feeding means and image forming means (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-255053). In this method, however, vapor generating means must be newly provided, and it is hard to control the humidity around the image forming means with the vapor generating means.
As another method of controlling the external environment around image forming means, for example, a method of applying liquid to a recording sheet before printing, and heating and drying the sheet at the time of printing to evaporate the liquid applied on the recording sheet, so that the humidity is increased near the head has been disclosed (see, for example, Japanese Patent Application Laid-Open No. 2003-165203).
In this method, however, since liquid is applied onto the recording sheet, the image quality is directly influenced, and nozzle clogging cannot be prevented unless printing is conducted frequently.
Another proposal is a method of using an atmosphere control box enclosing the atmosphere around the head, and sending gas adjusted in temperature and humidity into the atmosphere control box (see, for example, JP-A No. 2004-243164). In this method, the gas to be sent must be adjusted in temperature and humidity, adverse effects may be caused on ejection of the ink depending on the gas blowing direction, and the design of the apparatus is also difficult.
In the ink jet system, the ejection stability of droplets tends to be influenced by the external environment and, in particular, the nozzles tend to become clogged if not used for a long time in dry conditions.
To solve this problem, attempts have been made to cap the nozzle surface to shut off the outside air and suppress drying around the nozzles when not ejecting. However, when not used for a long time or when exposed to high temperature, drying of nozzles cannot be prevented sufficiently.
As the cap material, it has been proposed to use a moistened high water absorbing resin (see, for example, JP-A No. 6-64180). In this method, when dried, the high water absorbing resin gradually releases water, and it is thought that drying in the cap can thus be prevented.
However, high water absorbing resin absorbs water and is swollen, so that a cap made of the high water absorbing resin cannot be fixed to the jig, and when, on the other hand, the cap releases water and contracts, the airtightness of the cap is decreased.
Further, the cap must be moistened and then fixed before use.
As a method of preventing drying of nozzles, it has also been proposed to use a hydrophilic metal porous body for the cap (see, for example, JP-A No. 6-64180). In this method, a metal porous body having fine pores of tens to hundreds of μm suctions and holds ink by capillary action, so that drying in the cap is prevented.
In this method, however, a certain volume of ink must be suctioned by capillary action, so that it is necessary to install a container holding the ink, and connect the container to the end of the cap formed of the metal porous body. As a result, the cap structure is complicated, and at least the connecting portion to the container holding the moisture and a part of the cap must be formed of the metal porous body.